Blender 2.8: units of scale, grid, and default size of objects
Scene &gt; Units &gt; Units Scale $0.001$, Length &gt; Millimeters



Viewport Overlays &gt; Grid Scale $0.001$


File &gt; Defaults &gt; Save Startup File


Restart Blender, then
Add &gt; Mesh &gt; Cube, Torus, open the sidebar to check